Conventional photonic semiconductor devices, as represented by semiconductor lasers, are provided with a light-receiving element for monitoring the optical output of a light-emitting element, and the voltage applied to the light-emitting element is controlled based on the level of light detected by the light-receiving element so as to keep the optical output of the light-emitting element constant. FIG. 17 is a sectional view showing an example of a conventional photonic semiconductor device. In the photonic semiconductor device shown in FIG. 17, an n-type low concentration impurity layer (n− layer) 6 is laid on top of an n-type high concentration impurity layer (n+ layer) 5 to form a silicon substrate 2, and, just below the top surface of part of this silicon substrate 2, a diffusion layer (p+ layer) 6 diffused with a p-type impurity such as boron is formed to form a PIN-type photodiode, which functions as a light-receiving element 1. On the top surface of the silicon substrate 2, an insulating layer 8 of silicon oxide or the like is formed. Moreover, on top of the insulating layer 8, above the top surface of the silicon substrate 2 where the diffusion layer 3 is not formed, a light-emitting element mount electrode 10 is formed, on which a light-emitting element 18 is fixed with a conductive adhesive B such as Ag paste (see, for example, the publication of Japanese Patent Application Laid-Open No. H6-53603).
The conventional photonic semiconductor device structured as described above have been suffering from unwanted influence of electric charge, such as that produced by the voltage applied to the light-emitting element mount electrode 10, on the output of the light-receiving element 1.
Moreover, it has been suffering also from variation of the output Im of the light-receiving element 1 according to variation of the height H from the top surface of the insulating layer 8 to the light-emitting point (active layer) 18b of the light-emitting element 18. FIG. 18 shows the relationship between the height H and the output current Im of the light-receiving element 18. This diagram shows, for H=10 μm and 130 μm, the relationship between the distance L from the light-emitting point 18b of the light-emitting element to the light-receiving region 4 and the output current Im. When the height H equals 130 μm (solid line), the output current Im of the light-receiving element is constant irrespective of the distance L from the light-emitting point to the light-receiving regions; by contrast, when the height H equals as low as 10 μm (broken line), the output current Im of the light-receiving element decreases sharply as the distance L increases. On the other hand, to permit the heat produced at the light-emitting point to be rejected effectively through the semiconductor substrate, the light-emitting element needs to have as small a height H as possible. Thus, fitting the light-emitting element with the distance L as close to zero as possible has customarily been requiring high accuracy.
Furthermore, quite inconveniently, light-emitting elements based on gallium nitride or the like are prone to be destroyed by static electricity accumulated on a worker during the fabrication of devices incorporating them.